"Train up a child in the way he should go: and when he is old, he will not depart from it." (Proverbs 22:6)
This verse always confused me. I never had any doubts about the Bible and believed it completely...except for this verse. 
I thought of all the people I knew or had heard of who were raised in Christian homes but turned away from their early teachings. I thought, with so many people on the wrong path, even after their early teachings, how could this verse be true?  
But today something stood out to me about this verse, something I'd never noticed before. It doesn't say "when he's an adult, he will not depart from it". It doesn't say "when he's grown up, he will not depart from it". It specifically uses the word "when he is OLD, he will not depart from it".
So it got me thinking. 
When you instill these early teachings in your children, you have planted a seed inside them. 
Some seeds burst forth with fruit right away...others seeds stay underneath the dirt for a long time.  
But even a plant that grows slowly can still become the tallest tree- because it had strong roots underneath it!
When you plant seeds in your garden, you do all the work you can, but then you must wait patiently for the sun to pull them upward.  Likewise, when you plant seeds in your children, you do all the work you can, but then must wait patiently for the Son to pull them upward. 
This may not happen when they reach adulthood. Sometimes it happens as people enter the middle of their lives and stop finding meaning in the superficial glitters of the world around them. Sometimes it happens as people grow old and wonder what else there is beyond this world. Sometimes it happens on the death bed. 
Wherever and whenever it happens, God is faithful, loving, and forgiving. He never fails and He ALWAYS keeps His word. 
Parents who pour their energy, their hearts, their hopes and prayers and tears have invested in their children's hearts, and that work is never lost. 
Returning to the faith of their youth may happen soon, and you will be able to rejoice in that with them! 
Or, unfortunately, it may not happen until they're much, much older- maybe long after their parents are gone, maybe when they're old and have grandchildren of their own- but you can rest assured that it WILL happen. It WILL happen- because God promises us that it will happen!  "When he is old, he will not depart from it".